This project is the investigation of the cellular and molecular interactions during synaptogenesis, and is a continuation of the project described in the previous annual report. We have taken the interaction between neurons and muscle cells grown in vitro as a model system of the development of the neuromuscular junction. We have found that a factor or factors producted by neuronal cells, when added to muscle cells: 1) aggregates acetylcholine receptros (AChR), 2) causes the lateral immobility of AChR, 3) promotes the attachment of AChR to the myotube cytoskeleton, 4) decreases the rate of degradation of AChR.